Multiple forms of rat liver tyrosine aminotransferase can be readily discerned by CM-Sephadex chromatography. Three major peaks can be isolated in a stable form (they can be recycled without change in their electron pattern). The distribution of these various forms of the enzyme depends on a number of factors such as: ionic strength; the presence or absence of certain metabolic intermediates; and the presence or absence of particulate fractions of the cell. It is apparent that very precise conditions must be presented for any meaningful discussion of multiple forms of this enzyme. At present detailed procedures and conditions are being sought to determine which form will predominate. In adjunct studies of tyrosine aminotransferase regulation, a nucleolar endonuclease was discovered which may have an effect on induction of this enzyme by cortisol. At present, it is difficult to determine whether this endonuclease is affected by cortisol administration. Changes ranging from 2-30% have been observed. However, sufficient data has not been accumulated as yet. Further characterization of the enzyme is also underway.